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Query: EC:2.3.3.1 (citrate synthase)
 4,488 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Citrate synthase from Escherichia coli enhances the fluorescence of its allosteric inhibitor, NADH, and shifts the peak of emission of the coenzyme from 457 to 428 nm. These effects have been used to measure the binding of NADH to this enzyme under various conditions. The dissociation constant for the NADH-citrate synthase complex is about 0.28 muM at pH 6.2, but increases toward alkaline pH as if binding depends on protonation of a group with a pKa of about 7.05. Over the pH range 6.2-8.7, the number of binding sites decreases from about 0.65 to about 0.25 per citrate synthase subunit. The midpoint of this transition is at about pH 7.7, and it may be one reflection of the partial depolymerization of the enzyme which is known to occur in this pH range. A gel filtration method has been used to verify that the fluorescence enhancement technique accurately reveals all of the NADH molecules bound to the enzyme in the concentration range of interest. NAD+ and NADP+ were weak competitive inhibitors of NADH binding at pH 7.8 (Ki values greater than 1 mM), but stronger inhibition was shown by 5'-AMP and 3'-AMP, with Ki values of 83 +/- 5 and 65 +/- 4 muM, respectively. Acetyl-CoA, one of the substrates, and KCl, an activator, also inhibit the binding in a weakly cooperative manner. All of these effects are consistent with kinetic observations on this system. We interpret our results in terms of two types of binding site for nucleotides on citrate synthase: an active site which binds acetyl-CoA, the substrate, or its analogue 3'-AMP; and an allosteric site which binds NADH or its analogue 5'-AMP and has a lesser affinity for other nicotinamide adenine dinucloetides. When the active site is occupied, we propose that NADH cannot bind to the allosteric site, but 5'-AMP can; conversely, when NADH is the in the allosteric site, the active site cannot be occupied. In addition to these two classes of sites, there must be points for interaction with KCl and other salts. Oxaloacetate, the second substrate, and alpha-ketoglutarate, an inhibitor whose mode of action is believed to be allosteric, have no effect on NADH binding to citrate synthase at pH 7.8. When NADH is bound to citrate synthase, it quenches the intrinsic tryptophan fluorescence of the enzyme. The amount of quenching is proportional to the amount of NADH bound, at least up to a binding ratio of 0.50 NADH per enzyme subunit. This amount of binding leads to the quenching of 53 +/- 5% of the enzyme fluorescence, which means that one NADH molecule can quench all the intrinsic fluorescence of the subunit to which it binds.
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PMID:The binding of reduced nicotinamide adenine dinucleotide to citrate synthase of Escherichia coli K12. 0 77

We isolated and characterized mutants defective in nuo, encoding NADH dehydrogenase I, the multisubunit complex homologous to eucaryotic mitochondrial complex I. By Southern hybridization and/or sequence analysis, we characterized three distinct mutations: a polar insertion designated nuoG::Tn10-1, a nonpolar insertion designated nuoF::Km-1, and a large deletion designated delta(nuoFGHIJKL)-1. Cells carrying any of these three mutations exhibited identical phenotypes. Each mutant exhibited reduced NADH oxidase activity, grew poorly on minimal salts medium containing acetate as the sole carbon source, and failed to produce the inner, L-aspartate chemotactic band on tryptone swarm plates. During exponential growth in tryptone broth, nuo mutants grew as rapidly as wild-type cells and excreted similar amounts of acetate into the medium. As they began the transition to stationary phase, in contrast to wild-type cells, the mutant cells abruptly slowed their growth and continued to excrete acetate. The growth defect was entirely suppressed by L-serine or D-pyruvate, partially suppressed by alpha-ketoglutarate or acetate, and not suppressed by L-aspartate or L-glutamate. We extended these studies, analyzing the sequential consumption of amino acids by both wild-type and nuo mutant cells growing in tryptone broth. During the lag and exponential phases, both wild-type and mutant cells consumed, in order, L-serine and L-aspartate. As they began the transition to stationary phase, both cell types consumed L-tryptophan. Whereas wild-type cells then consumed L-glutamate, glycine, L-threonine, and L-alanine, mutant cells utilized these amino acids poorly. We propose that cells defective for NADH dehydrogenase I exhibit all these phenotypes, because large NADH/NAD+ ratios inhibit certain tricarboxylic acid cycle enzymes, e.g., citrate synthase and malate dehydrogenase.
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PMID:Mutations in NADH:ubiquinone oxidoreductase of Escherichia coli affect growth on mixed amino acids. 815 82

The murine small heat shock protein Hsp25 carries a single cysteine residue in position 141 of its amino acid sequence. Interestingly, Hsp25 can exist within the cell as covalently bound dimer which is linked by an intermolecular disulfide bond between two monomers. Oxidative stress caused by treatment of the cells with diamide, arsenite, or hydrogen peroxide leads to an increase in Hsp25-dimerisation which can be blocked by simultaneous treatment with reducing agents. Recombinant Hsp25 was prepared in an oxidized dimeric (oxHsp25) and reduced monomeric (redHsp25) from. The two species were compared with regard to secondary structure, stability, oligomerization properties and their chaperone activity. It is demonstrated by CD measurements in the far UV region that there are no significant differences in the secondary structure and temperature- or pH-stability of oxHsp25 and redHsp25. However, according to CD measurements in the near UV region an increase in the asymmetry of the microenvironment of aromatic residues in oxHsp25 is observed. Furthermore, an increase in stability of the hydrophobic environment of the tryptophan residues mainly located in the N-terminal domain of the protein against urea denaturation is detected in oxHsp25. Both reduced and oxidized Hsp25 from oligomeric complexes of similar size and stability against detergents and both species prevent thermal aggregation of citrate synthase and assist significantly in oxaloacetic acid-induced refolding of the enzyme. Hence, the overall secondary structure, the degree of oligomerization and the chaperone activity of Hsp25 seem independent of the formation of the intermolecular disulfide bond and only the stability of the hydrophobic N-terminal part of the molecule is influenced by formation of this bound. The obtained data do not exclude the possible involvement of dimerization of this protein in other cellular functions, e.g. in intracellular sulfhydryl-buffering or in the protection of actin filaments from fragmentation upon oxidative stress.
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PMID:The effect of the intersubunit disulfide bond on the structural and functional properties of the small heat shock protein Hsp25. 965 71

Urea-induced unfolding of Escherichia coli citrate synthase occurs in two phases, as monitored by circular dichroism at 222 nm (measuring secondary structure) or by tryptophan fluorescence. In this paper we characterize the intermediate state, which retains about 40% of the ellipticity of the native state, and is stable between 2.5 M and 5.5 M urea, approximately. This intermediate binds significant amounts of the probe for hydrophobic surfaces, anilinonaphthalene sulfonate, but forms aggregates at least as high as an octamer, as shown by transverse urea gradient polyacrylamide electrophoresis. Thermal denaturation of E. coli citrate synthase also produces an intermediate at temperatures near 60 degrees C, which also retains about 40% of the native ellipticity and forms aggregates, as measured by electrospray-ionization/time-of-flight mass spectrometry. We have used a collection of "cavity-forming" mutant proteins, in which bulky buried hydrophobic residues are replaced by alanines, to explore the nature of the intermediate state further. A certain amount of these mutant proteins shows a destabilized intermediate, as measured by the urea concentration range in which the intermediate is observed. These mutants are found in parts of the citrate synthase sequence that, in a native state, form helices G, M, N, Q, R, and S. From this and other evidence, it is argued that the intermediate state is an aggregated state in which these six helices, or parts of them, remain folded, and that formation of this intermediate is also likely to be a key step in the folding of E. coli citrate synthase.
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PMID:A stable intermediate in the equilibrium unfolding of Escherichia coli citrate synthase. 1033 22

NtHSP18P (HSP18), a cytosolic class I small heat-shock protein from tobacco pollen grains, was expressed in Escherichia coli. The viability of these cells was improved by 50% at 50 degrees C, demonstrating its functionality in vivo. Purified recombinant protein formed 240 kDa HSP18 oligomers, irrespective of temperature. These oligomers interacted with the model substrate citrate synthase (CS) to form large complexes in a temperature-dependent manner. Furthermore, HSP18 prevented thermally induced aggregation of CS at 45 degrees C. The fluorescence probe bis-ANS revealed the exposure of HSP18 hydrophobic surfaces at this temperature. Reactivation of chemically denatured CS was also significantly enhanced by HSP18. Surprisingly, HSP18 function was inhibited (in contrast to the related chaperone alphabeta-crystallin and plant sHSPs studied so far) by the presence of ATP in a concentration-dependent manner. The conformational changes of HSP18 imposed by ATP binding were indicated by the difference in the quenching of intrinsic tryptophan fluorescence, and implied more compact structure with ATP. Fluorescence measurements with bis-ANS showed that the conformational shift of HSP18 is suppressed in the presence of ATP. Decreased chaperone activity of HSP18 in the presence of ATP is caused by the lower affinity of conformationally blocked HSP18 for the substrate, as demonstrated by a higher susceptibility of model substrate, malate dehydrogenase, to proteolytic cleavage. Our results suggest that the chaperone activity of some plant sHSPs could be regulated by the availability of ATP in the cytoplasm, which would provide a mechanism to monitor the cell environment, control biological activity of sHSPs, and coordinate it with other ATP-dependent chaperones such as HSP70.
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PMID:Chaperone activity of tobacco HSP18, a small heat-shock protein, is inhibited by ATP. 1099 82

Experiments with mini-alphaA-crystallin (KFVIFLDVKHFSPEDLTVK) showed that Phe(71) in alphaA-crystallin could be essential for the chaperone-like action of the protein (Sharma, K. K., Kumar, R. S., Kumar, G. S., and Quinn, P. T. (2000) J. Biol. Chem. 275, 3767-3771). In the present study we replaced Phe(71) in rat alphaA-crystallin with Gly by site-directed mutagenesis and then compared the structural and functional properties of the mutant protein with the wild-type protein. There were no differences in molecular size or intrinsic tryptophan fluorescence between the proteins. However, 1,1'-bi(4-anilino)naphthalene-5,5'-disulfonic acid interaction indicated a higher hydrophobicity for the mutant protein. Both wild-type and mutant proteins displayed similar secondary structure during far UV CD experiments. Near UV CD signal showed a slight difference in the tertiary structure around the 285-295 region for the two proteins. The mutant protein was totally inactive in suppressing the aggregation of reduced insulin, heat-denatured citrate synthase, and alcohol dehydrogenase. However, a marginal suppression of beta(L)-crystallin aggregation was observed when mutant alphaA-crystallin was included. These results suggest that Phe(71) contributes to the chaperone-like action of alphaA-crystallin. Therefore we conclude that the 70-88-region in alphaA-crystallin, identified by us earlier, is the functional chaperone site in alphaA-crystallin.
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PMID:Phe71 is essential for chaperone-like function in alpha A-crystallin. 1159 24

Work in Saccharomyces cerevisiae has shown that Atp12p binds to unassembled alpha subunits of F(1) and in so doing prevents the alpha subunit from associating with itself in non-productive complexes during assembly of the F(1) moiety of the mitochondrial ATP synthase. We have developed a method to prepare recombinant Atp12p after expression of its human cDNA in bacterial cells. The molecular chaperone activity of HuAtp12p was studied using citrate synthase as a model substrate. Wild type HuAtp12p suppresses the aggregation of thermally inactivated citrate synthase. In contrast, the mutant protein HuAtp12p(E240K), which harbors a lysine at the position of the highly conserved Glu-240, fails to prevent citrate synthase aggregation at 43 degrees C. No significant differences were observed between the wild type and the mutant proteins as judged by sedimentation analysis, cysteine titration, tryptophan emission spectra, or limited proteolysis, which suggests that the E240K mutation alters the activity of HuAtp12p with minimal effects on the physical integrity of the protein. An additional important finding of this work is that the equilibrium chemical denaturation curve of HuAtp12p shows two components, the first of which is associated with protein aggregation. This result is consistent with a model for Atp12p structure in which there is a hydrophobic chaperone domain that is buried within the protein interior.
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PMID:The molecular chaperone, Atp12p, from Homo sapiens. In vitro studies with purified wild type and mutant (E240K) proteins. 1470 7

The formation of all major intermediates in the reaction catalyzed by the citrate synthase from Thermoplasma acidophilum is accompanied by changes in tryptophan fluorescence. The largest change is the strong quenching observed on formation of the binary complex with substrate, oxaloacetate (OAA). The four tryptophan residues present in the enzyme have been changed to nonfluorescent ones in various combinations without major perturbations in protein stability, enzyme mechanism, or other physical properties. W348, residing in the hydrophobic core of the protein behind the active site wall ca. 9 A from OAA, is responsible for the majority of the protein's intrinsic fluorescence and all of the quenching that accompanies OAA binding. Lifetime studies show that all of the quenching results from excited-state processes. The lack of solvent isotope effects on the quantum yields excludes a quenching mechanism involving proton transfer to an acceptor. There are no significant changes in fluorescence properties in single site mutants of residues near W348 that change conformation and/or interactions when OAA binds. This result excludes these changes from a direct role. Electron transfer from the indole excited state to some acceptor is the major quenching mechanism; the reduced quenching observed in the 5F-W-substituted protein strengthens this conclusion. Using the X-ray structures of the unliganded enzyme and its OAA binary complex, hybrid quantum mechanics-molecular dynamics (QM-MM) calculations show that OAA itself is the most likely quencher with the OAA carbonyl as the electron acceptor. This conclusion is strengthened by the ability of an alpha-keto acid model compound, trimethylpyruvate, to act as a diffusional quencher of indole fluorescence in solution. The theoretical calculations further indicate that the positive electrostatic potential surrounding the OAA carbonyl within the enzymes' active site is essential to its ability to accept an electron from the excited state of W348. These same environmental factors play a major role in activating OAA to react with the carbanion of acetyl-CoA. Since carbonyl polarization plays a role in the catalytic strategies of numerous enzymes whose reactions involve this functional group, tryptophan fluorescence changes might be useful as a mechanistic probe for other systems.
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PMID:Photophysics of tryptophan fluorescence: link with the catalytic strategy of the citrate synthase from Thermoplasma acidophilum. 1568 25

We reported previously that chemical modification of human alphaA-crystallin by a metabolic dicarbonyl compound, methylglyoxal (MGO), enhances its chaperone-like function, a phenomenon which we attributed to formation of argpyrimidine at arginine residues (R) 21, 49, and 103. This structural change removes the positive charge on the arginine residues. To explore this mechanism further, we replaced these three R residues with a neutral alanine (A) residue one at a time or in combination and examined the impact on the structure and chaperone function. Measurement of intrinsic tryptophan fluorescence and near-UV CD spectra revealed alteration of the microenvironment of aromatic amino acid residues in mutant proteins. When compared to wild-type (wt) alphaA-crystallin, the chaperone function of R21A and R103A mutants increased 20% and 18% as measured by the insulin aggregation assay and increased it as much as 39% and 28% when measured by the citrate synthase (CS) aggregation assay. While the R49A mutant lost most of its chaperone function, R21A/R103A and R21A/R49A/R103A mutants had slightly better function (6-14% and 10-14%) than the wt protein in these assays. R21A and R103A mutants had higher surface hydrophobicity than wt alphaA-crystallin, but the R49A mutant had lower hydrophobicity. R21A and R103A mutants, but not the R49A mutant, were more efficient than wt protein in refolding guanidine hydrochloride-treated malate dehydrogenase to its native state. Our findings indicate that the positive charges on R21, R49, and R103 are important determinants of the chaperone function of alphaA-crystallin and suggest that chemical modification of arginine residues may play a role in protein aggregation during lens aging and cataract formation.
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PMID:Effect of site-directed mutagenesis of methylglyoxal-modifiable arginine residues on the structure and chaperone function of human alphaA-crystallin. 1658 92

During aging, human lens proteins undergo several post-translational modifications, one of which is glycation. This process leads to the formation of advanced glycation end products (AGEs) which accumulate with time possibly leading to the formation of cataract. alphaB-Crystallin, a predominant protein in the lens, is a member of the small heat shock proteins (sHSPs) which are a ubiquitous class of molecular chaperones that interact with partially denatured proteins to prevent aggregation. This chaperone function is considered to be vital for the maintenance of lens transparency and in the prevention of cataract. In the present study, we introduced an analog of the advanced glycation end product, OP-lysine, at the 90th position of a mutated human alphaB-crystallin (K90C) by covalent modification of the cysteine residue with N-(2-bromoethyl)-3-oxidopyridinium hydrobromide. The AGE-modified K90C-alphaB-crystallin is termed as K90C-OP. We compared the structural and functional properties of K90C-OP with the original K90C mutant, with K90C chemically modified back to a lysine analog (K90C-AE), and with wild-type human alphaB-crystallin. Modified K90C-OP showed decreased intrinsic tryptophan fluorescence and bis-ANS binding without significant alterations in either the secondary, tertiary, or quaternary structure. K90C-OP, however, exhibited a reduced efficiency in the chaperoning ability with alcohol dehydrogenase, insulin, and citrate synthase as substrates compared to the other alpha-crystallin proteins. Therefore, introduction of a single AGE near the chaperone site of human alphaB-crystallin can alter the chaperoning ability of the protein with only minor changes in the local environment of the protein.
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PMID:Effect of a single AGE modification on the structure and chaperone activity of human alphaB-crystallin. 1802 13


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